In order to help differentiate omap_devices from normal
platform_devices, make them all a parent of a new common parent
device.

Then, in order to determine if a platform_device is also an
omap_device, checking the parent is all that is needed.

Users of this feature are the runtime PM core for OMAP, where we need
to know if a device being passed in is an omap_device or not in order
to know whether to call the omap_device API with it.

In addition, all omap_devices will now show up under /sys/devices/omap
instead of /sys/devices/platform
Acked-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>